We propose an approach for development of an HIV-1 envelope glyprotein immunogen that induced antibodies capable of potent neutralization of primary HIV-1 strains of multiple subtypes, as well as pathogenic, recombinant Simian/Human Immunodeficiency Viruses (SHIV). The work builds on previous efforts incorporating selected HIV-1 envelopes administered using in vivo replicon expression system and soluble, oligomeric gp140 as immunogens. Previous studies of two different envelopes in small animals have resulted in interclade cross-reactive primary virus neutralizing antibodies. One of these envelopes, the CD4-independent R2 envelope, has been studied in non-human primates. Monkeys that received immunogens incorporating this Env developed antibodies that neutralized most of a panel of primary HIV-1 viruses, including strains of heterologous subtypes, and SHIV DH12R Clone 7. SHIV DH12R Clone 7 is moderately pathogenic. Some monkeys challenged with that SHIV were protected from infection, while others had reduced levels and duration of virus replication. The pathogenic SHIV 89.6p and SF162p3 were neutralized poorly or not at all. In this application we propose to incorporate into our immunization regimen an HIV-1 Env that has been engineered to induce primary virus, interclade cross-reactive neutralizing antibodies with markedly different specificities than neutralizing antibodies induced by the R2 Env. We also propose an approach to induction of antibodies against conserved, cross-reactive neutralization epitopes that were probably not targeted effectively with the R2 immunogen. In parallel we propose approaches for more sustained in vivo expression or use of more effective adjuvant with soluble gp140. The changes in neutralization properties of pathogenic SHIV that resulted from in vivo SHIV adaptation will be studied and considered in our immunogen design. Our goal is to combine the advances proposed to form an immunogen which induces high level sterilizing immunity against pathogenic SHIV in rhesus monkeys.